Vitamins are defined as organic substances occurring in many foods in small amounts that are necessary for the normal metabolic functioning of the body. Vitamin D is one of the fat-soluble vitamins. Our bodies can't manufacture vitamins but can manufacture hormones, and some experts feel that vitamin D should be classified as a hormone rather than a vitamin because our bodies manufacture it when exposed to the sun.

"Current Medical Diagnosis and Treatment 2016" notes that vitamin D deficiency "is increasing throughout the world as a result of diminished exposure to sunlight caused by urbanization, automobile and public transportation, modest clothing and sunscreen use."

Defining vitamin D deficiency can be confusing due to two ways of measuring level: nmol/liter and ng/ml (e.g. 112 nmol/liter is the same as 50 ng/ml). The latter is the way levels are usually reported and is what will be used in today's column. Significant vitamin D deficiency is defined as a level less than 20 ng/ml and this occurs in 29 percent of postmenopausal American women and 25 percent of American men older than 65. Severe deficiency is defined as a level less than 10 ng/ml and is present in 3.5 percent of Americans.

Almost all cells, organs and tissues in our bodies have vitamin D receptors, and vitamin D can also turn on hundreds of genes. Over the years, vitamin D supplementation has been touted as a panacea for all sorts of health problems, but according to Dr. Michael Greger's website nutritionfacts.org, better studies done in the last few years have discounted many of these claims. Here's what the current science tells us about vitamin D deficiency:

Vitamin D promotes calcium absorption by the intestines, and also stimulates the activity of bone-forming cells called osteoblasts. Deficiency can cause osteoporosis and osteomalacia, which like osteoporosis causes brittle bones and fractures, but is not exactly the same as osteoporosis. In children with developing bones, osteomalacia is called rickets, which can result in permanent skeletal deformities.

There are vitamin D receptors in our muscles and nervous systems including our brains, but as we age the number of receptors decreases. Elderly people with low vitamin D levels are more apt to suffer falls due to muscle weakness and balance problems.

Vitamin D boosts our immune system, and people with low D levels have an increased incidence of respiratory infections.

Low vitamin D levels are associated with increased all-cause mortality (i.e. you live longer if you maintain normal vitamin D levels).

Vitamin D helps fight inflammation. Asthma, ulcerative colitis and Crohn's disease all inflammatory diseases improve and in some cases even go into remission once D levels have normalized.

According to Dr. Joel Fuhrman, author of "Eat to Live" and other books, "Vitamin D regulates several genes and cellular processes related to cancer progression." People with low levels of D are more apt to get several cancers including breast and colon, and once they get cancer it is more likely to progress.

What are normal levels? Greger points out that the cradle of civilization was in equatorial Africa, "when people were running around outside naked." Vitamin D levels in African tribes living traditional lifestyles are around 50. Breast milk lacks vitamin D, and therefore breast-fed babies are given D supplements, which doesn't make sense from an evolutionary point of view. But if a breast-feeding mother's D level is 50 or greater, her breast milk does contain vitamin D. So while some guidelines say we should shoot for levels of D greater than 30, most of the science points to levels of 50 or more as ideal.

How much D should people take to achieve levels of 50 or above? For most people, 2,000 units a day achieves optimal D levels, with some caveats:

Vitamin D is stored in fat, so obese people need to take 4,000 units a day a day to achieve optimal levels.

Absorption is hampered in the elderly, so the American Geriatrics Society recommends 4,000 units in people 65 and older.

The type of vitamin D you should take is D3, which is what your body makes when exposed to sunlight; versus D2 present in yeast and mushrooms, which isn't as effective.

D is absorbed better if taken with a meal that contains some fat, such as nuts and seeds.

The practice of taking very high doses (e.g. 50,000 units) intermittently is now frowned upon, because the very high levels that result can cause problems.

How about just getting sun exposure rather than taking a supplement? The problem is that sun ages your skin and causes skin cancer. When outside you should cover up and apply sunscreen to exposed areas of your skin such as your face, but this interferes with vitamin D production. Tanning booths have the same problems as sun exposure, and aren't very effective in vitamin D production anyway.

Should everyone have their vitamin D levels tested? Most guidelines don't recommend this because:

Almost all Americans are lower than optimal in vitamin D.

Most insurance companies and Medicare won't cover the test when coded as a screen.

The test for vitamin D is done on a blood sample, and is not a very accurate test in that a lot of variation can occur between labs and even on the same sample tested repeatedly in the same lab.

Vitamin D is inexpensive and has no side effects except in very high doses, such as 10,000 units a day, which can result in dangerously high blood levels.

Dr. Feinsinger, who retired from Glenwood Medical Associates after 42 years as a family physician, now has a nonprofit Center For Prevention and Treatment of Disease Through Nutrition. He is available for free consultations about heart attack prevention and any other medical concerns. Call 970-379-5718 for an appointment. For questions about his columns, email him at gfeinsinger@comcast.net.

Read more here:
Doctor's Tip: The low-down on vitamin D - Glenwood Springs Post Independent

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